In social psychology , an ingroup is a social group towards which an individual feels loyalty and respect, usually due to membership in the group. This loyalty often manifests itself as an ingroup bias. Commonly encountered ingroups include family members, people of the same race, culture or religion, and so on.

By contrast, an outgroup is a social group to which an individual does not identify. For example, people may find it psychologically meaningful to view themselves according to their race, culture, gender or religion. It has been found that the psychological membership of social groups and categories is associated with a wide variety of phenomena.

The terminology was made popular by Henri Tajfel and colleagues during his work in formulating social identity theory. The significance of ingroup and outgroup categorization was identified using a method called the minimal group paradigm. Tajfel and colleagues found that people can form self preferencing ingroups within a matter of minutes and that such groups can form even on the basis of seemingly trivial characteristics, such as preferences for certain paintings.[1][2][3]

This refers to the fact that under certain conditions people will preference and have affinity for one’s in-group over the out-group, or anyone viewed as outside the in-group. This can be expressed in evaluation of others, linking, allocation of resources and many other ways.[4]
Research demonstrates that people often privilege ingroup members over outgroup members even when the ingroup has no actual social standing; for instance, a group of people with the same color shirts, when the other group has another color of shirt.[5].

Discrimination between ingroups and outgroups is a matter of favoritism towards an ingroup and the absence of equivalent favoritism towards an outgroup [6]. Outgroup derogation is the phenomena in which an outgroup is perceived as being threatening to the members of an ingroup [7]. This phenomenon often accompanies ingroup favoritism, as it requires one to have an affinity towards their ingroup. Some research suggests that outgroup derogation occurs when an outgroup is perceived as blocking or hindering the goals of an ingroup. It has also been argued that outgroup derogation is a natural consequence of the categorization process [8] .

People have been shown to be differentially influenced by ingroup members. That is, under conditions where group categorization is psychologically salient people will shift their beliefs in line with ingroup social norms.

This generally refers to the tendency for groups to make decisions that are more extreme than the initial inclination of its members, although polarization toward the most central beliefs has also been observed. It has been shown that this effect is related to a psychologically salient ingroup and outgroup categorization

Categorization of people into social groups increases the perception that group members are similar to one another. An outcome of this is the outgroup homogeneity effect. This refers to the perception of members of an outgroup as being homogenous, while members of one's ingroup are perceived as being diverse, e.g. "they are alike; we are diverse,” [9][10]. This is especially likely to occur in regards to negative characteristics. Under certain conditions, ingroup members can be perceived as being similar to one another in regards to positive characteristics. This effect is called ingroup homogeneity. [11]

In evolutionary psychology, ingroup favoritism is seen as an evolved mechanism selected for the advantages of coalition affiliation.[12] It has been argued that characteristics such as gender and ethnicity inflexible or even essential features of such systems.[13][14]However, there is evidence that elements of favoritism are flexible in that they can be erased by changes in social categorization.[15] One study in the field of behavioural genetics suggests that biological mechanisms may exist which favor a coexistence of both flexible and essentialist systems.[16]